Anisotropic conductive films (ACF) are used to make electrical connections in miniature and high-performance electronic equipment that require fine and dense wiring implementations for connecting the terminals of circuit boards. Descriptions of such ACF films and their use can be found in U.S. Pat. No. 5,770,305 and United States Patent Application Publication No. 2002/0111055 A1, the entire disclosure of which are incorporated herein by reference.
Electrical connections made between glass substrates of display devices, such as, liquid crystal displays (LCD) and plasma display panels (PDP) are examples of ACF film applications. In these applications, ACF films are often used to electrically connect flexible wiring leads to one or more metallic terminal pads on the periphery of the display device's glass substrate. The flexible wiring leads are usually made of a flexible polymer and typically have thin metal foil wirings that terminate into ACF bonding pads on the surface of the flexible wiring leads.
FIG. 1 is a cross-sectional view of an ACF film 10 in a typical application. The ACF film 10 is positioned between an ACF bonding pad 20 of a flexible wiring lead 70 and a base terminal pad 30 of a glass substrate 80. The bonding pad 20 and the base terminal pad 30 are generally made of thin metal alloy foils (generally a copper based alloy). The ACF film 10 comprises conductive particles 14 that are coated with an insulation material and dispersed within a binder material 12. When the bonding pad 20 and terminal pad 30 are compressed together, squeezing the ACF film 10, the ACF film 10 acts as the bonding agent bonding the two metal pads together and at the same time also provides the electrical connection between the bonding pad 20 and the terminal pad 30. This bonding method will be referred to herein as pressure bonding.
FIG. 2 is a cross-sectional schematic illustration of the assembly of FIG. 1 after the pads 20 and 30 have been pressure bonded together by a compression force, represented by arrows 50. The electrical connection between the pads 20 and 30 is made by the conductive particles 14 dispersed within the ACF film 10. When the pads 20 and 30 are compressed together, the conductive particles 14 trapped between the pads are squeezed. The compression force 50 should be sufficiently high to crush and deform the conductive particles 14 breaking their coating of insulation material. Thus, metal-to-metal contacts are formed between the crushed conductive particles 14a and the bonding pad 20 and the base terminal pad 30 establishing electrical conduction paths between the pads 20 and 30.
In ACF film 10, the binder material 12 is the adhesive that bonds to the pads 20 and 30 to hold them together. But, when the pads 20 and 30 are compressed together to activate the conductive particles of the ACF film, much of the binder material 12 is squeezed out from between the pads 20 and 30, leaving behind very little binder material 12 between the pads 20 and 30. Thus, as illustrated in FIG. 3, once the compression force 50 is removed, the elasticity of the conductive particles 14a exert force on the pads 20 and 30 causing them to separate. But the abundantly present binder material 12 in the regions C, immediately adjacent to the pads 20 and 30, will keep the periphery regions A of the pads 20 and 30 from separating. Thus, the result is that the peripheral regions A of the bonding pads generally stay bonded together while the bonding pads 20 and 30 are separated in the center region. Therefore, the conductive particles 14a sandwiched between the bonding pads 20 and 30 near the peripheral regions A remain under sufficient compression to form good electrical conduction paths between the bonding pads 20 and 30.
The ACF bonding pads are generally thin metal foils formed on flexible wiring leads so in this example of a typical application, it would be the bonding pad 20, whose center region bows outward and separate away from the base terminal pad 30. Because the base terminal pad 30 is on a rigid glass substrate, it would not flex or bow. In some applications, ACF film may be used to bond to thin ACF bonding pads together. For example, two flexible wiring leads having ACF bonding pads on them may be connected together using an ACF film. In those cases, both of the two mating ACF bonding pads will bow out in the center region since they are both flexible.
Because the center regions of the bonding pads 20 and 30 have separated and are not exerting sufficient compression force on to the conduction particles 14b in the center region, the electrical connection between the bonding pads 20 and 30 are pretty much limited to the peripheral regions A. This problem is also illustrated in FIG. 4, which is a plan view microphotograph of the conventional ACF bonding pad configuration of FIG. 3. The view is taken from above the bonding pad 20. The outline of the conductive particles 14a remaining under compression in the peripheral regions of the bonding pad 20 can be seen through the bonding pad 20.
Thus, an enhanced bonding pad designs in this type of application is needed to improve the electrical contact between the bonding pads utilizing ACF films.